Hello!
MODS 3.4 XML Schema available at
http://www.loc.gov/standards/mods/mods.xsd defines
xs:simpleType name="languageAuthorityAttributeDefinition"
as an
xs:restriction base="xs:string"
containing (among others)
xs:enumeration value="rfc4646"
But RFC-4646 is now obsoleted and replaced by
http://tools.ietf.org/rfc/rfc5646.xml
which takes into account RFC-5645 containing some important improvements.
Therefore, I would suggest to slightly modify
http://www.loc.gov/standards/mods/mods.xsd
to support RFC-5646, which could be achieved with the additon of
xs:enumeration value="rfc5646"
in the definition of
xs:simpleType name="languageAuthorityAttributeDefinition"
Regards!
Saašha,